My pH meter is a PH-200, in the same price range than the Extech. Its readings are clearly wrong, it can go from say, pH 6 to pH 6.5 if the temperature of the liquid being tested drops 5C, despite having ATC and its temperature readings being spot on (checked with a thermometer I trust).Can this behaviour be caused by a bad probe, or do I have to blame the main unit and think about getting another pH meter?

Most often, lack of a stabilized reading or jumps are caused by clogged reference junctions. When protein, fat, or salts clog it up, the reference solution (usually KCl) cannot flow. And sometimes it will go the other way, where whatever you're measuring will flow back up into the electrode, fouling it up. What you need to do is follow the procedure I listed to try and clear up the junction.

Some junctions are better suited than others for dairy. For example, high quality ceramic or teflon tend to work very well and do not clog easily.

This is why proper care is so vital. If the protein gets caked in, no matter what you do, you may not be able to unclog the junction, and then you have to replace them or the probe.

One of my most trusty probes has a double junction and stabilizes really fast. I get a reading in under 10 seconds most of the time.

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Linuxboy - thanks for clearing this up. Except for the potassium chloride I seem to have everything (though I have rennet, not renin) and the rest of your instructions are clear. Thanks for following up on this.

so... Last night, following this discussion I fired it up to check on a new type of washed rind cheese I am working on. It's 24 hour old, (not yet salted). According to my calculations it should have been at 4.6 pH. I cleaned the probe well with warm water and detergent, then with rinse liquid. I then stuck it in the cheese (seriously, these poke marks are becoming part of the presentation aspect of my cheese). The reading was clear: 4.6pH wait, no 4.7 ahh, 5.1, 5.0, 4.6 again. Okay Most of the time the number was indeed 4.6 which was perfect. The fluctuations in the numbers seem to have been affected by movement of the probe and I needed to rest it for 30 seconds or so to stabilize the reafing. As I moved it I saw water moving in it (it's a clear probe where you can see the wires). It seems that water has penetrated it when I cleaned it (or some liquid is supposed to be there and I've never noticed it before???). I would assume this confirms that this probe is defective. Wouldn't you say?

I still don't know what's the deal with my Hanna meter but it's a cheapo meter so maybe I should get a new one. I think that a ExTech ExStik 100 or 110 may do the Trick. They have flat probe as I understand it, and no futzing around with a screw to move it to 7.00pH or 4.01pH when calibrating. Perhaps my meter is just old. The thing is that the flat food probe for my meter costs as much as a brand new ExTech 100 anyway so maybe I should just get that. I wish I could afford the Hanna HI 99161. $350 is more than I wanted to spend but it's a quick pH meter that is made especially for dairy.

But the real lesson is that I had a cheese arriving at its proper and exact acidity levels at the right time without using any pH meter to begin with. I think this is because I learned to "feel" (look, smell, taste) the milk, whey and curd. I pay attention to the room conditions and I time the cheesemaking stages correctly. The more I do it, it seems the higher my success rate is.

Yes, I know that if I want to get serious about producing the cheese or certifying others for some of my originals I must get used to using the meter for consistency and as a safety control point for HACCP purposes. I do love that I have eliminated any dependency on this instrument for production of quality cheese and I just keep it as a control and alert point. In flying it's called Pilotage - the skill to fly by visual landmarks and by feeling the aircraft and environment. You have to perfect it before you can move to instrument flying.

In any case... any recommendations on what meter I need to purchase? Anyone?

(it's a clear probe where you can see the wires). It seems that water has penetrated it when I cleaned it (or some liquid is supposed to be there and I've never noticed it before???). I would assume this confirms that this probe is defective. Wouldn't you say?

That's the reference solution inside the probe. Usually saturated KCl. Why do you think it's defective? There's no way to tell if it's fouled up except to take a sample of it and test for impurities.

Rennet IS mostly rennin. All I meant is you need an enzyme to clean the proteins. Use contact lens solution if you're unsure. Buy some KCl. It's cheap and makes for great storage solution and is useful in conditioning meters.

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I think it's defective because my readings are unstable and jump very quickly from 4.6 to 5.1, back to 4.7, 5.0 and 4.6 again etc. It feels very volition and unstable. Is that normal that the numbers keep changing on the screen when your probe is stuck in the cheese for 2 minutes already? The numbers don't change towards a stable reading but are just all over the place.

That jumping around is not necessarily a fouled reference solution. When the reference solution fouls up, the meter becomes uncalibrateable. It consistently reads a bias, or it will never fully stabilize. I think you have a clogged junction and need a good soak in acid and then KCl. Real fouling is somewhat rare. Clogs are far more common.

I like ISFET probes because they're easy to clean. If not then, then flat tip or spear ones. But those get so expensive. The Extech is a decent middle ground when it works. I don't like everything about it, but it does alright if you get a good one.

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If I use a pH meter I would do it for professional reasons and want one that is accurate, reliable and fast. It would be pointless to use any pH meter that cannot perform consistently. Being 0.2pH off is enough to mess up batches of otherwise good cheese. In other words, I need to trust it and make decisions based on the reading rather than just use it for logging.

My understanding is that ISFET probes do not need to be stored in solution ...but are very expensive.

Not true. That's just a marketing claim to get people to buy them. Yes, if you use the meter every day, and the junction stays hydrated, you should be OK with dry storage. Meaning, there's no bulb, so the issue of bulb hydration is moot. But if you're not using the meter regularly, then you must store in either KCl, or a KCl with 4 buffer in it.

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want one that is accurate, reliable and fast.

All meters have their quirks. And all meters require maintenance. Some more than others. And all meters fail, and all meters need their probe replaced with use because over time, there's drift. Not avoidable.

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probe cleaning solutions for dairy and cheese

This is HCl and an enzyme, such as pepsin. They use the same formulation I detailed. It is an industry standard to remove salts and proteins. The HCl also lowers the pH to activate the pepsin. If you soak in an enzymatic cleaner you make yourself, it does the same thing. IMHO, you can buy it if you want the assurance that it's "official". Sort of like having organic certification vs not when you know the underlying product is the same.

That Hanna is a pretty nice unit. It works OK. In my experience, my Oakton with ISFET stabilizes faster. If I plug in a regular double junction probe, it performs about the same. If you take care of the probes, most commercial meters work well (except cheap stick types). I'm personally a fan of using dedicated handheld units, and then replacing the probes every so often. Because then the probes run $100 or so. Of course, if you get a working Extech, their probe replacements are cheap... $50. And they also sell junctions for them.

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I thought they were accurate ...if you know what range to look for. You need the right paper for a small range of pH at every test. That means you would use something else for 5.5pH and for 4.6pH and that's a bit wasteful and very expensive.

That jumping around is not necessarily a fouled reference solution. When the reference solution fouls up, the meter becomes uncalibrateable. It consistently reads a bias, or it will never fully stabilize. I think you have a clogged junction and need a good soak in acid and then KCl. Real fouling is somewhat rare. Clogs are far more common.